It used to be that when planning a new community, planners would just build up housing without thought to the people who were going to live there let alone the environment. The goal of urbanization was to just create housing for the growing number people moving to cities for jobs. Things like grocery stores, gas stations, fast food places and even parks were an after thought. You can still see neighborhoods like that today (like mine).
The urban planners of today have better ideas for urbanization, ones that include parks. But one of the things urban planners have had a problem dealing with is rain run-off. During heavy rains and big floods (like what happened with Hurricane Katrina) rain water mixes with all the bad stuff that may be laying around like pesticides and even dog poop. Water treatment facilities can become swamped with contaminated water, sometimes so much so that they end up just passing it on through and into the nearest river instead of letting it back up into the public sewer.

Urban planners have begun building series of culverts, ponds and artificial wetlands to collect rain run-off so that water treatment facilities don’t become overloaded. The problem with the pools is that after a while they start growing algae. Some algae is not so bad, but eventually algae can kill a pond by sucking all the oxygen out of the water. Miklas Scholz, an environmental engineering researcher, started adding goldfish to experimental research ponds. The fish kept the ponds algae free and also make the ponds more aesthetically appealing.

The introduction of the goldfish may actually build up an ecosystem in these artificially built ponds. Fish do attract birds like cranes and there’s got to be some kind of animal that likes cranes and so forth. You know, its that circle of life thing.